Neutrons were discovered nearly a century ago, but still hold a few secrets. For example, a lone neutron can transform into other subatomic particles - a proton, an electron and an electron antineutrino - but efforts to measure just how long this decay takes have come up with different numbers.

Such decay times are fundamental in the "Standard Model" of physics, which aims to describe in detail how matter as we now know it came to be in the earliest moments of the Universe's history, and also shed light on the fusion happening for example in stars.

The Standard Model also suggests that despite having no net charge, there is a small separation of charges within neutrons that would give them what is known as an electric dipole moment - a kind of electric north and south pole. However, experiments have until now been too inexact to measure it.

A number of research efforts continue worldwide to pin down these basic properties, such as those at the Isis neutron source at the Rutherford Appleton Laboratory in the UK.

Numbers game

What these measurements have needed to gain more precision is, quite simply, a greater number of "ultra-cold" - or very slow-moving - neutrons to study.

Now Oliver Zimmer and colleagues working at the Institut Laue Langevin (ILL) in Grenoble, France have bottled up neutrons at a density of 55 per cubic centimetre - more than five times higher than the previous record - also at the ILL.

Neutrons produced in a reactor at the ILL escape at scorching speeds, and must be slowed significantly

As the highest-intensity neutron source in the world, the ILL puts the particles to work on topics ranging from gravity to medicine to the environment, and has a particular focus on the slow-moving variety.

The turbine used to slow neutrons before now has been the workhorse for their world-beating neutron density for some 26 years.

The new approach, first developed by Dr Zimmer and colleagues at the Technical University Munich in 2007 and reported in a paper in Physical Review Letters, has been refined at the ILL.

It uses superfluid helium-4 at a temperature of -269C - just four degrees above absolute zero - to slow the neutrons down, taming them toward the 55-per-cubic-centimetre benchmark.

However, Dr Zimmer said, "these are still scarily low numbers".

"Things often depend on statistical precision," he told BBC News. "The more particles you have, the more precise result you will get."

Although the current density is enough to start to tackle the big questions about neutrons, Dr Zimmer said he believes that the same approach could bring the neutron density to 1,000 per cubic centimetre.

"By increasing the precision of these experiments you can peek into this region where you can exclude theories beyond the Standard Model, or even find the signs of new physics," he said.